1. Field of the Invention
The present invention relates to an information signal processing apparatus for processing an information signal.
2. Description of the Related Art
As one example of an apparatus for processing a frequency-modulated information signal, there has heretofore been provided an apparatus, such as a video tape recorder, which is arranged to reproduce and demodulate an audio signal which is recorded on a magnetic tape in a frequency-modulated state.
FIG. 1 is a block diagram of a conventional information signal processing apparatus, and schematically shows one example of the arrangement of a demodulating apparatus for demodulating a frequency-modulated audio signal reproduced from a recording medium.
Referring to FIG. 1, a reproduced RF signal which has been reproduced from a recording medium (not shown), such as a magnetic tape, by a reproducing head is supplied to an analog/digital (A/D) converter 102 via a pre-filter 101.
The reproduced RF signal is a signal in which a four-frequency pilot signal 201 for tracking control, a modulated chrominance signal 202, a frequency-modulated audio signal 203 and a modulated luminance signal 204 are frequency-multiplexed as shown in FIG. 2(a) by way of example.
The pre-filter 101 of FIG. 1 has a characteristic which is selected to extract only a frequency band 205 in which the frequency-modulated audio signal 203 is positioned from the input reproduced RF signal as shown in FIG. 2(b) by way of example.
The A/D converter 102 performs sampling of the signal supplied from the pre-filter 101, in accordance with a clock signal of predetermined frequency generated by a clock generator 103, thereby forming and outputting a sample pulse signal.
The clock signal generated by the clock generator 103 is also supplied to each of the other parts in the demodulation processing block shown as a dashed-line frame in FIG. 1.
It is to be noted that the processing frequency of a demodulating operation for demodulating the frequency-modulated audio signal in a manner which will be described below is the sampling frequency of the sampling operation performed by the A/D converter 102 in accordance with the clock signal generated by the clock generator 103.
For example, if the frequency-modulated audio signal has a carrier frequency of 1.5 MHz and a frequency deviation of .+-.100 KHz, it is necessary to set the sampling frequency of the A/D converter 102 to a frequency greater than or equal to twice the maximum value of the frequency component contained in a signal to be demodulated (i.e., the frequency-modulated audio signal). Therefore, the sampling frequency needs to be greater than or equal to 3.2 MHz ((1.5 MHz+100 KHz).times.2=3.2 MHz), for example, 4 MHz including a small margin.
If the signal outputted from the pre-filter 101 is sampled by the A/D converter 102 at the aforesaid sampling frequency of 4 MHz, a signal having the frequency spectrum shown in FIG. 2(c) is outputted from the A/D converter 102.
In FIG. 2(c), reference numeral 206 denotes the sampling frequency, and reference numeral 207 denotes the Nyquist frequency of the sampling frequency. The frequency 207 is 1/2 of the sampling frequency 206.
The sample pulse signal outputted from the A/D converter 102 is transmitted along two different paths, and the sample pulse signal transmitted along one path is directly supplied to a divider 105, while the sample pulse signal transmitted along the other path is supplied to the divider 105 after being phase-shifted by .pi./2 by a phase shifter 104.
If the sample pulse signal is represented as "A sin.theta.(t)" ("A" represents its amplitude), the signal passed through the phase shifter 104 is represented as "A cos.theta.(t)". The divider 105 performs a division using the supplied two signals and outputs a signal indicative of "A sin.theta.(t)/A cos.theta.(t) =tan.theta.(t)", and a signal indicative of ".theta.(t)" is obtained by passing the signal indicative of "A sin.theta.(t)/A cos.theta.(t)=tan.theta.(t)" through a tan.sup.-1 circuit 106.
If ".omega.c" is the angular frequency of the carrier of the frequency-modulated audio signal and "f(t)" is the frequency-demodulated audio signal, ".theta.(t)" is represented as ".theta.(t)=.omega.ct+.intg.f(t)dt". Therefore, "d.theta.(t)/dt=.omega.c+f(t)" is obtained by passing the signal indicative of ".theta.(t)" through a differentiator 107. Then, a corrector 108 corrects ".omega.c" which corresponds to a D.C. component, and outputs the frequency-demodulated audio signal "f(t)".
However, the arrangement of the conventional demodulating apparatus described above has a number of disadvantages in that the frequency of the clock signal for controlling the operation of each of the parts in the demodulation processing block shown as the dashed-line frame in FIG. 1 is selected to be equal to the sampling frequency which is a frequency greater than or equal to twice the maximum value of the frequency component contained in the signal to be demodulated (i.e., the frequency-modulated audio signal). For example, since the amount of computational processing per unit time is large and it is, therefore, necessary to perform high-speed computational processing, the apparatus needs to be provided with a high-speed computational processing circuit which is extremely costly. If a digital signal processor or the like is used to construct the demodulation processing block, it will be extremely difficult to realize demodulation processing using software.
As one example of an apparatus for processing an information signal, there has heretofore been provided an apparatus which is arranged to reproduce a frequency-modulated information signal from a recording medium, on which the information signal is recorded in a frequency-modulated state, and demodulate the reproduced frequency-modulated information signal, as well as which has the function of detecting a dropout occurring in a reproduced frequency-modulated information signal owing to a defect or the like of the recording medium.
As is known in the art, the conventional information signal processing apparatus utilizes, for example, a method of detecting the occurrence of a dropout in a frequency-modulated information signal reproduced from a recording medium by detecting the envelope of the reproduced frequency-modulated information signal.
FIG. 3 is a block diagram of a conventional information signal processing apparatus, and schematically shows the arrangement of a so-called tan-type frequency-demodulating apparatus which is arranged to reproduce a frequency-modulated information signal from a recording medium and demodulate the reproduced frequency-modulated information signal, as well as which has the function of detecting the occurrence of a dropout in the reproduced frequency-modulated information signal.
Referring to FIG. 3, a reproduced frequency-modulated information signal which has been reproduced from a recording medium (not shown), such as a magnetic tape, by a reproducing head is transmitted into two different paths, and the signal transmitted along one path is directly supplied to a divider 302, while the signal transmitted along the other path is supplied to the divider 302 after being phase-shifted by .pi./2 by a phase shifter 301.
If the frequency-modulated information signal is represented as "A sin.theta.(t)" ("A" represents its amplitude), the signal passed through the phase shifter 301 is represented as "A cos.theta.(t)". The divider 302 performs a division using the supplied two signals and outputs a signal indicative of "A sin.theta.(t)/A cos.theta.(t)=tan.theta.(t)", and a signal indicative of ".theta.(t)" is obtained by passing the signal indicative of "A sin.theta.(t)/A cos.theta.(t)=tan.theta.(t)" through a tan.sup.-1 circuit 303.
If ".omega.c" is the angular frequency of the carrier of the frequency-modulated information signal and "f(t)" is the frequency-demodulated audio signal, ".theta.(t)" is represented as ".theta.(t)=.omega.ct +.intg.f(t)dt". Therefore, "d.theta.(t)/dt=.omega.c+f(t)" is obtained by passing the signal indicative of ".theta.(t)" through a differentiator 304. Then, a corrector 305 corrects ".omega.c" which corresponds to a D.C. component, and outputs the frequency-demodulated information signal "f(t)".
The reproduced frequency-modulated information signal which has been reproduced from the recording medium (not shown), such as a magnetic tape, by the reproducing head is also supplied to an envelope detecting circuit 306.
The frequency-modulated information signal supplied to the envelope detecting circuit 306 is converted into an absolute value by an absolute-value circuit 307 and the absolute value is supplied to a hold circuit 308. The supplied absolute value is subjected to envelope detecting processing on the basis of a time constant determined by a coefficient "K" set in a coefficient multiplier 309.
FIGS. 4(a) to 4(d) are views showing signal waveforms formed by the respective parts in the envelope detecting circuit 306. If a frequency-modulated information signal having the waveform shown in FIG. 4(a) is supplied, the waveform of the frequency-modulated information signal is formed into the waveform shown in FIG. 4(b) by the absolute-value processing performed by the absolute-value circuit 307. The waveform shown in FIG. 4(b) is formed into the signal waveform shown by a solid line in FIG. 4(c) by the hold processing performed by the hold circuit 308.
The hold processing performed by the hold circuit 308 will be described below in detail with reference to FIG. 4(d).
The signal having the waveform, shown in FIG. 4(b), which has been subjected to the absolute-value processing in the absolute-value circuit 307 is supplied to a comparator 310 and a switch 311. In the comparator 310, the level of the presently supplied signal is compared with the level of a signal which was supplied previous to the presently supplied signal at an interval of a predetermined time set by the delay time of a delay element Z.sup.-1.
If the level of the presently supplied signal is greater than the level of the signal which was supplied the predetermined time before, the switch 311 is connected to the terminal "a" shown in FIG. 3 so that the presently supplied signal is outputted to the succeeding stage via the delay element Z.sup.-1 (an interval A in FIG. 4(d)). If the level of the presently supplied signal is smaller than the level of the signal which was supplied the predetermined time before, the switch 311 is connected to the terminal "b" shown in FIG. 3 so that the level of the presently supplied signal is gradually attenuated on the basis of the time constant determined by the coefficient "K" set in the coefficient multiplier 309 and the resultant signal of attenuated level is outputted to the succeeding stage via the delay element Z.sup.-1 (an interval B in FIG. 4(d)). Incidentally, in FIG. 4(d) dashed lines indicate the waveform of the signal outputted from the absolute-value circuit 307.
The operations of constituent elements 312 to 316 of FIG. 3 will be described below with reference to FIGS. 5(a) to 5(c).
FIGS. 5(a) to 5(c) are views showing the relationships between the signal subjected to the hold processing outputted from the hold circuit 308, the level comparison signal outputted from the comparator 312 which will be described later, and a dropout detection signal outputted from the comparator 316 which will be described later. FIG. 5(a) shows the waveform of a signal which is obtained by passing a frequency-modulated information signal in which a dropout has occurred through the absolute-value circuit 307 and the hold circuit 308, FIG. 5(b) shows the waveform of the level comparison signal outputted from the comparator 312, and FIG. 5(c) shows the waveform of the dropout detection signal outputted from the comparator 316.
The signal which has been subjected to the hold processing in the hold circuit 308 of FIG. 3 is supplied to the comparator 312, in which the level of the supplied signal is compared with a reference level (corresponding to a level 501 in FIG. 5(a)) generated by a reference level generator 313. For example, if the level of the signal supplied from the hold circuit 308 is greater than the reference level, the comparator 312 outputs, as the level comparison signal, data indicative of "1" to a counter 314 provided at the succeeding stage. If the level of the supplied signal is smaller than the reference level, the comparator 312 outputs data indicative of "0" to the counter 314 as the level comparison signal (refer to FIG. 5(b)).
During the above-described operation, a clock pulse signal is supplied to the counter 314 from a clock generator 315. The counter 314 counts the number of pulses of the clock pulse signal supplied from the clock generator 315, during the interval that the data indicative of "0" is outputted from the comparator 312 (during an interval 502 in FIG. 5(b)). The counter 314 outputs count data indicative of the count value to the comparator 316 provided at the succeeding stage.
In the counter 316, the count data outputted from the counter 314 is compared with reference value data generated by a reference value data generator 317. For example, if the count data supplied from the counter 314 is smaller than the reference value data, the comparator 316 outputs data indicative of "0" as the dropout detection signal. If the count data is greater than the reference value data, the comparator 316 outputs data indicative of "1" as the dropout detection signal (refer to FIG. 5(c)) and resets the counting operation of the counter 314.
More specifically, in the above-described case, the count value obtained by causing the counter 314 to count the number of pulses during an interval indicated by 503 in FIG. 5(b) corresponds to the reference value data generated by the reference-value data generator 317, and an interval indicated by 504 in FIG. 5(c) is detected as the interval during which the dropout has occurred in the reproduced frequency-modulated information signal.
However, the arrangement of the above-described conventional information signal processing apparatus has a number of disadvantages. For example, it is necessary to provide a plurality of comparators as well as a reference level generator and a reference value data generator both of which serve to set parameters based on which the respective comparators perform their comparing operations. In addition, various circuits, such as an absolute-value circuit for performing absolute-value processing and a hold circuit for performing hold processing, are needed separately from a demodulating circuit for performing demodulation processing. As a result, a complicated, extremely costly circuit arrangement is needed.
As one example of an apparatus for subjecting an information signal to frequency-modulation processing and recording the frequency-modulated information signal on a recording medium, there has heretofore been provided an apparatus, such as a video tape recorder, which is arranged to subject an audio signal to frequency-modulation processing and record the audio signal on a magnetic tape in a frequency-modulated state.
FIG. 6 is a block diagram of a conventional information signal processing apparatus, and schematically shows one example of the arrangement of a frequency-modulating apparatus for subjecting an audio signal to frequency-modulation processing.
Referring to FIG. 6, an input audio signal is supplied to an analog/digital (A/D) converter 601. The A/D converter 601 performs sampling of the input audio signal, in accordance with a clock signal of predetermined frequency generated by a clock generator 604, thereby forming and outputting a sample pulse signal.
The clock signal generated by the clock generator 604 is also supplied to each part in a modulation processing block 602 shown as a dashed-line frame in FIG. 6. In the modulation processing block 602, the sample pulse signal is subjected to frequency-modulation processing which will be described later. The resultant signal is converted into an analog signal by a digital/analog (D/A) converter 603 which is also supplied with the clock signal generated by the clock generator 604, and the analog signal is outputted from the D/A converter 603.
It is to be noted that the frequency of the clock signal supplied from the clock generator 604 to each of the circuit blocks is set so that the frequency band of the frequency-modulated audio signal formed by the frequency-modulating apparatus becomes smaller than or equal to the Nyquist frequency.
For example, if the frequency-modulated audio signal has a carrier frequency of 1.5 MHz and a maximum frequency deviation of .+-.100 KHz, it is necessary to set the frequency of the clock signal generated by the clock generator 604 to a frequency greater than or equal to twice the maximum value of the frequency component contained in the frequency-modulated audio signal. Therefore, the frequency needs to be greater than or equal to 3.2 MHz ((1.5 MHz+100 KHz).times.2=3.2 MHz), for example, 4 MHz including a small margin.
The frequency-modulating operation performed in the modulation processing block 602 of the frequency-modulating apparatus shown in FIG. 6 will be described below.
Referring to FIG. 6, a sample pulse signal "f(t)" outputted from the A/D converter 601 is multiplied by a coefficient "c1" in a coefficient multiplier 605, so that a sample pulse signal "c1.multidot.f(t)" is outputted from the coefficient multiplier 605. In a coefficient adder 606, a coefficient "c2" is added to the sample pulse signal "c1.multidot.f(t)", so that a sample pulse signal "c2+C1.multidot.f(t)" is outputted from the coefficient adder 606.
Then, the sample pulse signal outputted from the coefficient adder 606 is integrated by an integrator 607, whereby a sample pulse signal ".intg.(c2+c1.multidot.f(t))dt=c2.multidot.t+c1.multidot..intg.f(t)dt" is formed. The sample pulse signal ".intg.(c2+c1.multidot.f(t))dt=c2.multidot.t+c1.multidot..intg.f(t)dt" is modulated into a sample pulse signal "sin(c2.multidot.t+c1.multidot..intg.f(t)dt)" which corresponds to the frequency-modulated audio signal, by a sin function circuit 608, and the sample pulse signal "sin(c2.multidot.t+c1.multidot..intg.f(t)dt)" is supplied to the D/A converter 603 provided at the succeeding stage.
However, the arrangement of the above-described conventional frequency-modulating apparatus has a number of disadvantages in that the frequency of the clock signal for controlling the operation of each of the parts in the modulation processing block 602 shown as the dashed-line frame in FIG. 6 is set to a frequency greater than or equal to twice the maximum value of the frequency component contained in the frequency-modulated audio signal. For example, since the amount of computational processing per unit time is large and it is, therefore, necessary to perform high-speed computational processing, the apparatus needs to be provided with a high-speed computational processing circuit which is extremely costly. If a digital signal processor or the like is used to construct the modulation processing block 602, it will be extremely difficult to realize modulation processing using software.
FIG. 8 shows another example of the conventional frequency-modulating apparatus. As shown, a preprocessing circuit 802 and other circuit elements are provided at a stage preceding the modulation processing block 602. An A/D converter 801, to which an audio signal is supplied, and the preprocessing circuit 802 are arranged to operate in accordance with a clock signal of frequency 40 KHz which is formed, for example, by dividing the frequency of a clock signal of frequency 4 MHz outputted from the clock generator 604 by a frequency divider 804. In such an arrangement, it is necessary to insert an interpolation filter 803 between the preprocessing circuit 802 and the modulation processing block 602 in order to increase the sampling frequency of the sample pulse signal outputted from the preprocessing circuit 802. However, the frequency of the operating clock signal of each of the A/D converter 801 and the preprocessing circuit 802 (i.e., 40 KHz) and the frequency of the operating clock signal of the modulation processing block 602 (i.e., 4 MHz) exhibit a large ratio (i.e., 1:100). As a result, the circuit scale of the interpolation filter 803 becomes large to incur an increase in cost.